1. Field of the Invention
The invention relates to an integrated hydraulic transaxle (IHT); i.e., IHT incorporating a hydrostatic transmission (HST) and an axle, and preferably, a mechanical deceleration drive train interposed between the HST and the axle.
2. Related Art
There is a well-known conventional hydraulic transaxle, i.e., IHT, which has a transmission casing integrally incorporating an HST, an axle, and a mechanical deceleration drive train (gearing) interposed between the HST and the axle. The HST comprises a hydraulic pump and motor fluidly connected to each other, wherein the hydraulic pump and/or motor has a movable swash plate for changing its displacement.
Reduction of the deceleration ratio of a mechanical deceleration drive train in an IHT is advantageous in minimizing and simplifying the IHT and reducing noise generated from the IHT. If the deceleration ratio of the mechanical deceleration drive train in an IHT is reduced, the HST is requested to increase its deceleration ratio between the hydraulic pump and motor so as to compensate for the reduction of deceleration ratio of the mechanical deceleration drive train. U.S. Pat. No. 5,201,692, for example, discloses a conventional IHT having an HST, in which the internal displacement of hydraulic pump is smaller than the internal displacement of hydraulic motor so as to increase the deceleration ratio of the HST.
In a usual HST, internal displacements of the pump and motor are equal to each other so that the maximum rotary speeds of the pump and motor are equal to each other. In this state, reciprocation of pistons in a cylinder block of the pump is synchronous to reciprocation of pistons in a cylinder block of the motor, thereby generating large vibration and noise. In the HST disclosed in the document, the difference of internal displacement between the hydraulic pump and the motor, such as not to synchronize the rotational frequency of the pump to that of the motor, is also advantageous to reduce vibration and noise.
In the IHT disclosed in the above document, the axial piston type hydraulic pump is provided with a cradle type movable swash plate. The cradle type movable swash plate has an arcuately convex surface slidably fitting to an arcuately concave inner surface of the IHT housing or a retainer fitted therealong. The concave surface of the IHT housing or the retainer must be so long as to ensure the sufficient slide range of the swash plate, thereby expanding the IHT housing. Further, the concave surface must be processed to resist strain, thereby increasing the manufacturing cost of the IHT. The fitting of surfaces between the swash plate and the IHT housing is subject to contamination and causes frictional resistance to reduce the efficiency of operational force for moving the swash plate. Thus, the disclosed IHT involves some obstacles in the way of being reduced in size and costs while ensuring its durability.
Furthermore, in the IHT disclosed in the above document, the hydraulic motor extends a motor shaft relatively rotatably penetrating a center section on which the hydraulic pump and motor are mounted and fluidly connected to each other through a closed fluid circuit therein. The peripheral surface area of the motor shaft slidably rotatably contacting the center section or a thrust bearing therebetween is considerably large so as to reduce the efficiency of transmitting power from the hydraulic motor to the motor shaft.
Such a loss of transmission efficiency is disadvantageous to minimize the HST because the HST must have such a large displacement as to absorb the loss.